Circuit boards that can accept a pluggable tab module that can be attached or removed without solder

ABSTRACT

The present invention relates to a circuitized board having removable flexible modules disposed thereon. The flexible modules are mechanically and electrically connected to the board to provide an apertured connect, yet may be easily removed without using heat. The invention also relates to a circuit package including a substrate and a flexible module. The substrate includes a circuit board, a plurality of pads affixed to the circuit board, and a plurality of gold plated metal balls affixed atop the pads. The flexible module includes a flexible dielectric member and gold plated vias disposed in the flexible dielectric member wherein at least some of the vias are positioned over the metal balls so that a line contact is made between the vias and the metal balls.

BACKGROUND OF THE INVENTION

In one typical process for manufacturing circuit boards, TAB and ATABmodules are attached to the circuit board by way of melting solder.Typically, the solder is a solder paste. The solder is reflowed at themelting point to provide the connection. Unfortunately, reflowtemperatures thermally stress the various heat sensitive modules andshorten the life span of the components. Eliminating a reflow step isdesirable.

Moreover, modules that are conventionally connected to solder boards arenot readily removed. If a defective module requires replacement, thesolder must be reflowed which creates thermal stress on the board andother components. Furthermore, TABs and ATABs are usually flexiblecomponents and present additional difficulties when connecting them tothe rigid circuit board. While cold welding has occasionally beenemployed, it frequently requires indium alloys at high pressures such as2,000 to 6,000 psi. Cold welded indium alloys typically corrode at theweld junction. Indium is also difficult to plate smoothly in thinlayers.

Accordingly, it would be desirable to attach modules to a board withoutheat or indium and to be able to remove such modules without heat.

SUMMARY OF THE INVENTION

The present invention relates to a circuitized board having removableflexible modules disposed thereon. The flexible modules are mechanicallyand electrically connected to the board to provide an apertured connect,yet may be easily removed without using heat. The invention also relatesto a circuit package and method of forming the package which packagecomprises a substrate and a flexible module. The substrate comprises acircuit board, a plurality of pads affixed to the circuit board, and aplurality of gold plated metal balls affixed atop the pads. The flexiblemodule comprises a flexible dielectric member and gold plated viasdisposed in the flexible dielectric member wherein at least some of thevias are positioned over the metal balls so that a line contact is madebetween the vias and the metal balls.

The invention also relates to a process for attaching a demountableflexible module to a circuit board, and includes the following steps:providing a circuit board having at least one, preferably a plurality,of gold plated metal balls disposed thereon; providing a flexible modulehaving at least one, preferably a plurality, of gold plated viasdisposed therein; positioning an elastomer stop over the circuit board;positioning the flexible module over the balls; applying pressure to theflexible module for a time sufficient to form a line contact between thegold plating on the vias and the gold plating on the metal ball of thecircuit board, wherein the module is removable without heating themodule. The process does not use solder to attach the module to theboard, and requires substantially less than 2000 psi, and preferablyless than 400 psi, to attach the module which produces little, if any,deformation to the flexible module.

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1 is a cross section of a circuit package depicting a flexiblemodule attached to a substrate;

FIG. 2 is an exploded view of FIG. 1 showing the individual componentsassembled in the process of making the circuit package;

FIG. 3 is a detail of FIG. 1 showing a metal ball; and

FIG. 4 is a detail of FIG. 3 showing the intermetallic areas below themetal ball.

DETAILED DESCRIPTION OF THE INVENTION

The present invention provides a circuitized board having removableflexible modules disposed thereon, which flexible modules are notattachedby solder. Rather, the flexible modules are mechanically andelectrically connected to the board, yet may be easily removed withoutusing heat. The invention also relates to a process for attachingflexible modules to a circuit board which does not use the metallizationof the solder or heat to attach the module to the board, and requiressubstantially less than 2000 psi, and preferably less than 400 psi, toattach the module. The process produces little if any deformation to themetallization of the vias in the flexible module.

Referring to FIG. 1, the package 10 is comprised of the substrate 12 andthe flexible module 14 disposed on the substrate 12.

The Substrate

The substrate 12 is comprised of a circuitized board 16 having metalballs 18 disposed on pads 20. The metal balls 18 are composed of metalcore 22 over which a first layer of nickel 24 and then a second layer ofgold 26 have been plated, as shown in FIG. 3. The second plate layer 26,which is gold, is preferably a minimum of 15 micro-inches thick. Thegold plating 26 can be thicker but this will add to the cost. The firstplated layer 24is preferably nickel; chromium is not suitable becausethe balls will not adhere to board. Other unsuitable materials for thefirst plated layer 24 include tin, iron, and nickel-iron alloy. In thepreferred embodiment, thesolder 28 combines with gold of gold layer 26which combines the nickel of nickel layer 24 to produce an intermetallicarea 29 of tin-gold-nickel, which has good adhesion to core 22, as shownin FIG. 4. The intermetallic area 29 slows the solder 28 from climbingbeyond metal line 31 which is also composed of tin, gold and nickel, andcovering ball 18 with solder. Intermetallic area 35 is rich in lead, andintermetallic area 37 is rich in gold and tin.

The composition of the metal core 22 is not critical as long as themeltingpoint is at least 200° C. higher than the melting point of thesolder 28 used to attach the balls 18 to the circuit board 16. Copper isthe preferred metal for the core 22 of the balls 18.

Conventional techniques are used to attach the metal balls 18 to thepads 20. For example, solder paste 28 is screened onto the pads 20.Preferably,a solder paste 28 comprised of 63% tin and 37% lead known as"63/37 solder paste" is used. Other solder pastes having differentratios or other metals are also suitable. Next, the metal balls 18 arepositioned onto thesolder paste 28 by conventional means. Typically, theballs 18 are placed onto special fixtures such as a ball carrier, andthe fixture is positioned over the circuit board 16 so that the ballscontact the solder paste 28.

Once the balls 18 are in position on the solder paste 28 on the circuitboard 16, the solder 28 is reflowed by conventional means, preferably inan oven at a temperature of about 160° C. to 250° C., preferably about180° C. to 200° C., for the 63/37 solder paste. The fixture, if present,is removed and then the metal balls 18 arecleaned to remove any oxidesor dirt on the surface.

The Flexible Module

The flexible module 14, shown in FIGS. 1, is preferably a polymer tapeautomated bonding also referred to herein as an "ATAB" module or a tapeautomated bonding referred to herein as a "TAB" module. The module 14comprises a flexible dielectric member 15, preferably a polyimide suchas is sold under the trade name Kapton®. At least one preferably aplurality, of vias 30 are disposed through the flexible dielectricmember 15. The walls 32 and pads 33 of the plated through holes 30 arecomprised of at least two layers, and preferably three layers, of metal.The outermost layer of metal on the walls 32 and pads 33 is gold. Thepresenceof at least two layers of metal helps to reduce stress at theinterface of the plated metal and the dielectric to prevent the viawalls from being ripped out of the vias 30. Preferably, the walls 32 andpads 33 of the vias 30 are plated with a layer of copper 34 to which alayer of nickel 36is applied. The outermost layer 38 plated over thenickel layer 36 is gold;preferably the gold is 99.9% pure. The gold mustbe essentially chromium-free to prevent the solder from climbing ontothe ball. The package 10 is further comprised of an elastomeric stop 40disposed over the circuit board 16. Elastomeric stop 40 has holes 42disposed therein toaccommodate pads 20. Disposed over the flexiblemodule 14 is elastomeric pad 44 which also has holes 46 to accommodateI/C chips 48. Chips 48 are connected preferably by either wire bondingor solder ball bonding. Groundplane 50 is preferably disposed on thebottom side, that is the side facingcircuit board 16, of flexibledielectric member 15. The vias 30 have a smaller diameter than the balls18. Preferably, the flexible module is notattached to any rigid memberwhich would impose rigidity on the flexible dielectric member 15.

Attaching the Flexible Module to the Substrate

First, as shown in FIG. 2, elastomeric "stop" 40 is positioned on thecircuit board 16; the elastomer "stop" 40 is preferably a sheet ofelastomeric material that has cutouts 42 that surround the balls 18. Theelastomeric stop 40 preferably has a maximum height that does notproject above the center line of balls 18. Next, a flexible module 14 ispositioned over the balls 18 so that the vias 30 in the module 14 arelocated above the balls 18.

Next, the elastomeric pressure pad 44 is placed on top of the flexiblemodule 14. Preferably, the pad 44 has holes 46 punched out toaccommodate any chips 48 disposed on the flexible module 14. Thepressure pad 44 distributes uniform pressure along the top of theflexible module 14. Preferably, the pressure pad 44 is made of anelastomeric material. Good results have been obtained using a sheet ofsilicone rubber, preferably having a durometer value of about 60. It ishighly preferred that the pressure pad 44 have a lower durometer valuethan the elastomeric stop

Next, a device for applying pressure such as a frame, is attached sothat the pressure pad 44 is pressed down over the module 14. Suitableframes include, for example, such frames as disclosed as elements 33 and51 in U.S. Pat. No. 4,902,234, to Brodsky et. al., issued Feb. 20, 1990and U.S.Pat. No. 5,009,393, to Bentlage et. al., issued Mar. 24, 1992,both of which are fully incorporated herein by reference. The pressureon the module 14 presses the vias 30 into contact with the gold platedballs 18 so that a line contact is made. As the balls 18 initiallycontact the via edge 45, the balls 18 slide over the edge 45 in a wipingaction which removes surface contaminants such as oxides. A gold-to-goldcontact is made after the wiping action, which results in cold fusionbonding of the gold of ball 18 to the gold of the wall 45 and/or surface32. Preferably at least 10 g/per ball is the minimum pressure applied toeach ball and preferably the maximum is 200 g/per ball.

The via walls and pads are rigid and not distorted when the line contactismade between balls 18 and the via walls and pads. Instead, theflexible dielectric member 15 is distorted or wrinkled as a result ofthe contact. That is, the balls 18 and vias 30 in achieving alignment,cause the flexible dielectric member 15 to buckle, to provide wrinkles52. The wrinkles 52 permit the flexible module 14 to expand as thecircuit board expands in response to heat; particularly thermal cycling.Elastomeric stop 40 supports the wrinkles 52 The elastomeric stop 40reduces the downward deflection of the module 14, specifically wrinkles52, when the pressure is applied. The elastomer stop 40 also preferablyacts as an insulator between the ground plane 50 of the module 14 andthe board 16.

Thus, during the assembly process and while the module 14 and board 16are under pressure, the balls 18 are not distorted as occurs withconventionalrigid modules. In conventional rigid modules balls distortto compensate for the variation in the size of the balls 18, theflatness of the board surface and/or the soldering process of the balls18 to the board 16. Instead in the package 10, these tolerances arecompensated by the flexureof the module 14 and the elastomer pressurepad 44 and elastomeric stop 40.The frame and pad 44 preferably remain onthe structure 10 although they may be removed.

Modules 14 attached by this method are physically connected andelectrically connected to the circuit board 16.

Removing the Module

First, the frame and elastomer pad 44 are removed to permit access tothe module 14. The module 14 is removed such as by using a tweezers topeel the module 14 away. Preferably, a replacement module is then placedover the balls 18 and the pad 44 and frame reassembled on the structure10. Theattachment method provides for simple and easy removal offlexible module that are defective or have a defective chip.

EXAMPLE 1

A 3/16ths of an inch thick glass-epoxy multilayer circuit board havingpadswas provided 63/37 solder paste, from 0.005" to 0.010" thick, wasscreened onto the pads. Balls having a diameter of 0.0075 inches werepositioned ina conventional locating fixture which was then positionedover the circuit board. The balls had 10 to 20 micro inch layer ofnickel plated on copper core and a 15 micro inch minimum layer of goldplated onto the nickel. Theballs were deposited onto the solder pastewhich has a melting point of about 160° C. The board was placed in aconventional oven at about 200° C. for about 90 seconds to reflow thesolder. The board was removed from the oven and allowed to cool. Anelastomeric stop composed ofa sheet of silicone rubber of about 90durometer 6 mils thickness, and having holes punched out in a patterncorresponding to the arrangement of the pads on the circuit board, wasfitted onto the circuit board. The balls projected above the top surfaceof the elastomeric stop. A flexible ATAB having vias of 0.006 inches indiameter, was positioned over the balls so that the balls and the viaswere aligned. The vias were copper over plated with 10 to 20 microinches of nickel over which was plated a minimum of 15 micro inches ofgold. An elastomeric pad made of 6 mils silicone rubber sheet having adurometer value of 90 was placed over the module. An all pad contactframe as disclosed in as elements 33 and 51 in U.S. Pat. No. 4,902,234,to Brodsky et. al., issued Feb. 20, 1990 and U.S.Pat. No. 5,009,393, toBentlage et. al., issued Mar. 24, 1992, was assembled around/over thecircuit board-module-pad and the screws tightened, using a torque wrenchto 2 inch pounds. The frame remained on the board module package for 120minutes.

The frame and pad were then removed to evaluate the circuits. Themodules attached were physically connected and electrically connected tothe circuit board, as determined by circuit testing. The circuit testingwas done with a test light in a conventional manner.

The flexible module was then removed by hand using a tweezers and adifferent flexible module was attached to the board at the samelocation. The frame was again assembled and removed. The new module wasthen tested and the electrical connections verified.

Although one embodiment of this invention has been shown and described,various adaptations and modifications can be made without departing fromthe scope of the invention as defined in the appended claims.

What is claimed is:
 1. A circuit package comprising:a. a substratehaving:a circuit pattern; at least one pad disposed on the circuitboard; at least one gold plated metal ball disposed atop said pad; b. aremovable flexible module electrically connected to the substrate,having:a flexible dielectric member having wrinkles therein; at leastone nondeformable gold plated via disposed through the flexibledielectric member, wherein said at least one gold plated via is disposedover the gold plated metal ball and electrically and structurallyconnected to the gold plated metal ball; and c. an elastomeric stopdisposed on the substrate.
 2. The circuit package of claim 1 whereinsaid at least one gold plated metal ball comprises a layer of golddisposed over a layer of nickel plate disposed over a metal core.
 3. Thecircuit package of claim 1 wherein the at least one ball and the atleast one plated via are substantially free of indium.
 4. The circuitpackage of claim 1 wherein the at least one ball and the at least oneplated via are substantially free of chromium.
 5. The circuit package ofclaim 1 wherein the via in contact with the solder ball is notsubstantially deformed.
 6. The circuit package of claim 1 wherein thereare at least two vias in the flexible module and at least two goldplated balls on the circuit board.
 7. The circuit package of claim 1wherein there are a plurality of vias in the flexible module and aplurality of gold plated balls on the circuit board.
 8. The package ofclaim 6 wherein the dielectric is a polymer tape automated bondingmodule or tape automated bonding module.
 9. The package of claim 6wherein the dielectric is a polyimide.
 10. The package of claim 6wherein the vias are gold plated.
 11. The package of claim 7 wherein thedielectric is a polyimide.